Hot plate unit

ABSTRACT

A hot plate unit ( 1 ) having a reduced heater temperature rise time and not generating dusts. The hot plate unit comprises a casing ( 2 ) having an opening portion; a heater ( 3 ) arranged on the opening portion and including a plate-shaped member ( 9 ) made from ceramics and a heat generating element ( 10 ) arranged on the plate-shaped member; and a radiant heat reflecting member ( 4 ) interposed between the casing and the heater.

BACKGROUND OF THE INVENTION

The present invention relates to a hot plate unit having a casing and aheater.

In a semiconductor production process, there is performed a step inwhich a photosensitive resin is formed as an etching resist on a siliconwafer, and this silicon wafer is then etched by using an etchant. Theapplication of the photosensitive resin onto the silicon wafer iscarried out by using a coating apparatus such as a spin coater. In thiscase, since the applied photosensitive resin is in a liquid andunhardened state, a drying step is first performed to lower the fluidityof the resin to some extent, and the resin is then subjected to anexposure/development step.

As an apparatus for drying the silicon wafer, which has undergone thecoating step, a hot plate unit, which employs a metal heater comprisingan aluminum plate having the back surface on which a heating element isarranged, has been used. However, in the prior art hot plate unit, themetal heater must inevitably be thicken to prevent the generation ofdistortion due to thermal expansion, and hence, it is poor in point oftemperature control.

Moreover, when the heater is heated, the casing is heated owing toradiant heat generated from the back surface of the heater, resulting inthe rise of the temperature of the casing. That is, thermal energy whichshould inherently be used for heating the heater is partially used toheat the casing, resulting in the loss of the thermal energy. Thisincreases a time required for increasing the heater temperature up to apredetermined temperature, which prolongs a time for the entire dryingstep, whereby the improvement of productivity tends to be disturbed. Inaddition, the casing is heated up to a temperature exceeding a heatresistance level of a metal material for the casing, which is notpreferable.

To solve the aforementioned problem, there can be considered a methodfor filling a heat insulating material such as a glass fiber between theheater and the casing so as to shut out the radiant heat. This mayprevent the excessive temperature rise of the casing, but the heatinsulating material generates dusts, deteriorating an ambientenvironment. Therefore, such an apparatus is not appropriate for asemiconductor production field which requires a highly cleanenvironment.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a hot plate unithaving a reduced heater temperature rise time and not generating dusts.

To solve the aforementioned problems, a first aspect of the presentinvention provides a hot plate unit comprising a casing having anopening portion; a heater arranged on the opening portion and includinga plate-shaped member made from ceramics and a heat generating elementarranged on the plate-shaped member; and a plate-shaped reflectionmember interposed between the casing and the heater.

The plate-shaped reflection member is preferably at least one selectedfrom a group consisting of a metal plate, a ceramic plate and a resinplate.

The ceramics are preferably nitride ceramics or carbide ceramics.

The plate-shaped reflection member is preferably arranged via apredetermined distance from the back surface of the heater and inparallel to the back surface.

The plate-shaped reflection member preferably comprises a plurality ofplate-shaped members.

The plate-shaped reflection member preferably has reflection planes onboth the sides thereof, respectively.

The plate-shaped reflection member preferably comprises a layerreflection member formed on an inner wall of the casing.

The plate-shaped reflection member preferably comprises a foilreflection member.

The casing and the heater are preferably arranged via a predetermineddistance from each other.

The heater preferably has a plurality of terminal pins electricallyconnected to the heat generating element, and a plurality of dummy pinswhich are longer than the terminal pins and which are not concerned inconduction with the heat generating element.

The heater is preferably equipped with a sleeve which receives theterminal pins and has heat resistance and insulating properties.

The plate-shaped member preferably comprises a plurality of plate-shapedmembers, and at least one heat generating element is interposed betweenat least one pair of plate-shaped members.

A second aspect of the present invention provides a hot plate unit whichcomprises a casing having an opening portion; a heater arranged on theopening portion and including a first plate-shaped member made fromceramics and a heat generating element arranged on the firstplate-shaped member; and a second plate-shaped member interposed betweenthe casing and the heater.

The second plate-shaped member is preferably at least one selected froma group consisting of a metal plate, a ceramic plate, and a resin plate.

The ceramics are preferably nitride ceramics or carbide ceramics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a hot plate unit according toa first embodiment of the present invention.

FIG. 2 schematically shows a pattern of a heat generating wiring layerof a heater constituting the hot plate unit.

FIG. 3 is a plan view of a stainless steel plate constituting the hotplate unit.

FIG. 4 is a graph showing a result of a performance test.

FIG. 5 is a partial sectional view of the hot plate unit.

FIG. 6 is a partial sectional view of another example of the hot plateunit.

FIG. 7 is a partial sectional view of still another example of the hotplate unit.

FIG. 8 is a partial sectional view of a further example of the hot plateunit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A hot plate unit 1 according to a first embodiment of the presentinvention will now be described with reference to FIGS. 1 to 5.

As shown in FIG. 1, the hot plate unit 1 includes a casing 2, a heater3, and a stainless steel plate 4 as a plate-shaped reflection member.

The casing 2 is a metal (here, aluminum) member having a bottom and has,at its upper portion, an opening portion 5 whose cross section iscircular. At a center portion of a bottom portion 2 a of the casing 2,three pin insert holes 6 are formed for receiving wafer support pins(not shown). By raising and lowering the wafer support pins inserted inthe pin insert holes 6, it is possible to move a wafer to a convey unitor receive the wafer from the convey unit. Moreover, at an outercircumference of the bottom portion 2 a, several lead line holes 7 areformed for passing a lead line. Lead lines 8 for supplying current tothe heater 3 are inserted into the holes 7.

In the first embodiment, a high-temperature heater 3 is preferably usedfor drying a silicon wafer (an object to be heated) coated by aphotosensitive resin at a high temperature (not lower than 500° C.).This heater 3 is composed of a plate-shaped member 9 formed fromceramics and a heat generating wiring layer 10 as a heat generatingobject or a heat generating element, and the heater 3 is arranged in theopening portion 5 of the casing 2.

The plate-shaped member 9 constituting the heater 3 is circular whosediameter is designed to be almost identical to the diameter of theopening portion 5 of the casing 2. The plate-shaped member 9 has alayered structure having a plurality of layers. The heat generatingwiring layer 10 is interposed between the layers. That is, the heatgenerating wiring layer 10 is not exposed at all from the upper surfaceof the heater 3.

More specifically, the ceramic material constituting the plate-shapedmember 9 is preferably nitride ceramics or carbide ceramics because ofthe following reasons. The nitride ceramics and the carbide ceramicshave excellent heat resistance, thereby to make a set temperature higherand reduce a time required for raising the temperature up to the settemperature level. Moreover, the nitride ceramics and the carbideceramics have thermal expansion coefficients smaller than metals, andaccordingly, neither warp nor distortion is generated by heating evenwhen the plate has a small thickness. Consequently, these materials arepreferable for reducing the thickness and weight of the heater 3.Furthermore, since the nitride ceramics and the carbide ceramics havehigh thermal conductivity values, the surface temperature of the heater3 can rapidly follow a temperature change of the heat generating wiringlayer 10.

The nitride ceramics are preferably, for example, metal nitride ceramicssuch as aluminum nitride, silicon nitride, boron nitride, or titaniumnitride.

The carbide ceramics are preferably metal carbide such as siliconcarbide, zirconium carbide, titanium carbide, tantalum carbide, tungstencarbide, and the like, for example. Among these ceramics, aluminumnitride is particularly preferred because it has the highest thermalconductivity, i.e., 180 W/m·K.

Here, the plate-shaped member constituting the heater 3 has preferably athickness of 0.5 mm to 5 mm and more preferably approximately 1 mm to 3mm. When the thickness is too small, the plate-shaped member 9 is easilybroken and when the thickness is too large, the hot plate unit maybecome larger in size and the production cost may be increased,disadvantageously.

As shown in FIG. 1, at a center portion of the heater 3, pin insertholes 11 are formed at three locations corresponding to the pin insertholes 6. Moreover, at the back surface of the heater 3, a plurality ofsets of two types of pins (terminal pins 12 and dummy pins 13) areprovided.

The terminal pins 12 related to the heat generating wiring layer 10 arearranged in two rows from the center portion toward the outercircumference of the heater 3. Each of these terminal pins 12 has abottom portion soldered to the land of a through hole 14 formed on theback side of the plate-shaped member 9. As a result, the terminal pins12 are electrically connected to the heat generating wiring layer 10. Itshould be noted that the bottom portions of the terminal pins 12 maydirectly be inserted into and engaged with the through holes 14. Thelead line 8 has a metal portion soldered to the tip end of the terminalpins 12. Accordingly, electric current is supplied via the lead line 8and the terminal pins 12 to the heat generating wiring layer 10 and as aresult, the temperature of the heat generating wiring layer 10 isincreased to heat the heater 3. It should be noted that the terminalpins 12 should have electric conductivity and accordingly, they are madefrom a conductive metal material such as Cobal and 42-alloy. In thefirst embodiment, such a conductive metal material is also used for thedummy pins 13 which do not require electric conductivity.

The dummy pins 13, which are not related to electric conductivity withthe heat generating wiring layer 16, are arranged at a plurality ofpositions of an outer circumference of the heater 3 and not connected tothe lead line 8. In the first embodiment, the dummy pins 13 are formedlonger than the terminal pins 12. In the first embodiment, each of thedummy pins 13 has a length of 30 mm while each of the terminal pins 12has a length of 17 mm. Accordingly, when the heater 3 is arranged at theopening 5 of the casing 2, only the tip end of each of the dummy pins isin contact with the outer circumference of the inner surface of thebottom portion 2 a of the casing 2. That is, the heater 3 is supportedin the horizontal direction by the dummy pins 13. Here, it is preferablethat a certain vacant space 15 be assured between the outercircumference of the back side of the heater 3 and the upper surface ofthe opening 5 of the casing 2. Thus, when the heater 3 is arranged insuch a non-contact state, it is possible to prevent increase of thetemperature of the casing 2 by the heat conduction from the heater 3.

It should be noted that the aforementioned heater 3 may include athermocouple buried therein, if necessary. In this case, it is possibleto control the temperature of the heater 3 by changing a voltage valueand a current value according to the temperature of the heater 3measured by the thermocouple.

As schematically shown in FIG. 2, in the heater 3, the heat generatingwiring layer 10 formed between the layers are patterned intoapproximately coaxial circles. This pattern has been employed touniformly heat the entire region of the heater 3, thereby reducing asmuch as possible, a temperature difference in the heater 3 as well as atemperature difference of a silicon wafer. The heat generating wiringlayer 10 is formed by sintering metal particles contained in aconductive paste.

The heat generating wiring layer 10 preferably has a thickness of 1 μmto 20 μm and a width of 0.5 mm to 5 mm. The heat generating wiring layer10 has a resistance value which can be changed by changing the thicknessand the width thereof, and the aforementioned ranges are most practical.

The conductive paste generally used contains a metal particle, a resin,a solvent, and a viscosity-increasing agent.

As a preferable metal particle to be used in the conductive paste, therecan be exemplified gold, silver, platinum, palladium, lead, tungsten,nickel, and the like. These metals are not easily oxidized even exposedto a high temperature and has a sufficient resistance value upon heatgeneration by electric conductance. The metal particle preferably has adiameter in a range of 0.1 μm to 100 μm. This is because when the metalparticle diameter is too small, oxidization is easily caused. On thecontrary, when the particle diameter is too large, sintering cannot beeasily performed and the resistance value becomes larger.

As a preferable resin used in the conductive paste, there can beexemplified an epoxy resin and a phenol resin. The solvent maypreferably include isopropyl alcohol and the viscosity-increasing agentmay preferably include cellulose.

In addition to the metal particles, the conductive paste preferablycontains a metal oxide. The reason is that such conductive paste enablesaccurate adhesion of the plate-shaped member 9 formed from ceramics tothe heat generating wiring layer 10 formed from metal, therebypreventing peeling off between the layers.

As the metal oxide, it is preferable to use, for example, lead oxide,zinc oxide, silica, boron oxide (B₂O₃), alumina, yttria, titania, andthe like. These oxides can improve the adhesion characteristic betweenthe metal and the ceramics without increasing the resistance value ofthe heat generating member.

As shown in FIG. 1 and FIG. 3, the reflection member used is aplate-shaped circular stainless steel plate 4 having a reflection planeS1 of radiant heat. This stainless steel plate 4 is designed to have adiameter smaller than that of the opening of the casing 2 and the heater3. At a center portion of the stainless steel plate 4, three pin insertholes 16 are formed at locations corresponding to the aforementioned pininsert holes 6. Moreover, the stainless steel plate 4 is also providedwith pin insert holes 17 for inserting the terminal pins 12 and thedummy pins 13. When assembling the apparatus, the terminal pins 12 areinserted into a sleeve 18 and then inserted into the pin insert holes 17of the stainless steel plate 4. The sleeve 18 is made from a ceramicmaterial (alumina and the like) having heat resistance and insulationcharacteristic so as to function to prevent contact between the terminalpins 12 and the stainless steel plate 4.

The stainless steel plate 4 has a radiant heat reflection plane S1 atleast on one side thereof. The reflection plane is referred to as asurface where radiant heat from a predetermined direction is reflectedunlike the absorption/transparency surface. Especially in the firstembodiment, the stainless steel plate 4 used has such a reflection planeS1 on both sides thereof.

The stainless steel plate 4 is arranged between the casing 2 and theheater 3 with a predetermined space distance L between the back surfaceof the heater 3 and the stainless steel plate 4 and in parallel to thecasing 2 and the heater 3. The predetermined space distance L1 ispreferably 3 mm to 20 mm, and more preferably 5 mm to 10 mm. In thefirst embodiment, the space distance L1 is set at 8.5 mm.

The reflection plane S1 is arranged so as to face the back surface ofthe heater 3 because of a reason as follows. The thermal energy loss isreduced by reflecting the radiant heat from the heater 3 and returningback the heat to the heater 3. In the first embodiment, since thereflection plane S1 is provided on both sides, this condition issatisfied. It should be noted that a predetermined space distance isassured between the bottom portion 2 a of the casing 2 and the stainlesssteel plate 4.

A spacer (not shown) made from heat-resistant ceramics may be providedbetween the upper surface of the stainless steel plate 4 and the backsurface of the heater 3. When such a spacer is provided, it is possibleto maintain the stainless steel plate 4 and the heater 3 in parallel toeach other while maintaining the space distance L1. Such a spacer ispreferably attached to the stainless steel plate 4 and to the heater 3using a heat-resistant adhesive.

Next, explanation will be given on an example of a procedure forproducing the hot plate unit 1.

A mixture is prepared by adding a sintering-promoting agent such asyttria and binder, if necessary, to a carbide or nitride ceramicspowder. This mixture is uniformly kneaded by using a three-roll mill andthe like. The resulting kneaded mixture is formed into an unprocessedmolded product having a sheet shape and approximately square shape(so-called green sheet) by using a doctor blade apparatus. The sheet mayalso be formed by using the press forming method as follows. That is,the aforementioned mixture is formed into particles by using the spraydrying method and the particles obtained are placed into a metal mold soas to be pressed, thereby forming an unprocessed molded product having asheet shape and approximately square shape. More specifically, in thefirst embodiment, sheet molding was performed using as a material akneaded mixture comprising 100 parts by weight of aluminum nitridepowder (average particle diameter: 1.1 μm), 4 parts by weight of yttria(yttrium oxide having an average particle diameter of 4 μm), 12 parts byweight of acrylic binder, and alcohol.

After preparing the necessary number of the unprocessed molded productsas layers, punching or drilling is performed to form holes for formingthrough holes and the pin insert holes 11. Furthermore, a conductivepaste prepared in advance is printed to fill the through-hole-formingholes so as to form through holes at predetermined positions. Afterthis, the conductive paste is printed onto the unprocessed moldedproduct by the screen printing method so as to obtain a predeterminedpattern. Then, the conductive paste forms the heat generating wiringlayer 10. Next, it is preferable that the conductive paste be dried toremove the solvent and binder contained in the paste. More specifically,in the first embodiment, the conductive paste was used for forming theheat generating wiring layer that contains tungsten.

Next, a plurality of the unprocessed molded products following theprinting step are layered, dried, pre-sintered, and complete-sintered ata predetermined temperature for a predetermined period of time. Thus,the unprocessed molded products and the conductive paste are sinteredsimultaneously and completely. As a result, it is possible to obtain theceramic plate-shaped member 9 having the heat generating wiring layer 10arranged as an inner layer thereof. The sintering step is preferablyperformed by using an HIP apparatus. When nitride ceramics or carbideceramics are used, the temperature is preferably set in a range fromapproximately 1500 to 2500° C. More specifically, in the firstembodiment using the nitride aluminum unprocessed molded products, theHIP was performed at a temperature of 1800° C. and under a pressure of230 kg/cm² and a sintered member (plate-shaped member 9) having athickness of 3 mm was obtained.

Subsequently, the plate-shaped member 9 is cut into a circular shape ofa predetermined diameter (230 mmφ in the first embodiment) and subjectedto a surface grinding process using a buff polishing apparatus or thelike. Then, the pins 12 and 13 are soldered onto the land of the throughholes 14. This completes the heater 3.

The pins 12 and 13 of the heater 3 are inserted into the pin insertholes 17 of the stainless steel plate 4 which has been prepared inadvance. The terminal pins 12 are further inserted into a sleeve 18 andin this state, the lead line 8 is soldered to the tip ends of therespective terminal pins 12. The heater 3 having the stainless steelplate 4 on the back thereof is mounted on the opening 5 of the casing 2.This completes assembling of the hot plate unit 1.

Next, explanation will be given on a method and results of a performancetest of the hot plate unit 1.

In this performance test, temperature increase and decreasecharacteristics are checked during heating of the heater 3. For thisperformance test, a thermocouple of the heater side is arranged at thecenter portion of the front surface of the heater 3 and a thermocoupleof the reflection member is arranged at the center portion of the backsurface of the stainless steel plate 4, and their temperatures arerespectively measured at a predetermined time interval. It should benoted that electric current is applied to the heater 3 for 24 minutesand after this, heating of the heater 3 is stopped for natural cooling.The heater 3 is set at approximately 550° C., at which a hold time isset to 20 minutes. FIG. 4 shows the test results.

In FIG. 4, the vertical axis represents the temperature (°C.) and thehorizontal axis represents time lapse (minute). A curve C1 showstemperature change of the front surface of the heater 3 while a curve C2shows temperature change of the back surface of the stainless steelplate 4.

When current application to the heater 3 is started, the front surfaceof the heater 3 is abruptly increased and reaches almost the settemperature of 550° C. in 4 or 5 minutes. The temperature of the heater3 is almost constant for 24 minutes after heating start. On the otherhand, temperature of the back surface of the stainless steel plate 4increases comparatively slowly and reaches only 120 or 130° C. when 5minutes have passed after the heating start. This temperature issuppressed to as low as approximately 230° C. even when 24 minutes havepassed after the heating start. Accordingly, the temperature differencebetween C1 and C2 is as large as 320° C. When current application to theheater 3 is stopped, the temperature of the front surface of the heater3 starts to lower and returns to the normal temperature after time lapseof about 15 minutes. The back surface temperature of the stainless steelplate 4 is also lowered likewise. Thus, in the first embodiment, aseries of steps is completed for about 40 minutes in total.

Moreover, a curve C3 in FIG. 3 shows a temperature change of a backsurface of an alumina plate replacing the stainless steel plate 4. Sincethe alumina has a low thermal conductivity as compared to the stainlesssteel, temperature increase of the back surface is suppressed.

Moreover, temperature distribution (difference between a highesttemperature and a lowest temperature) of the heating surface when thehot plate unit using the alumina reflection plate heated to 600° C. wasmeasured by using a thermo-viewer (IR162012-0012) produced by JapanDatum Co., Ltd. The temperature difference was 7° C. In the case of thehot plate unit using the stainless steel reflection plate, thetemperature difference was 10° C. Both of them showed preferable values.The difference in the temperature distribution caused by difference ofthe material used is considered to be caused by distortion of thereflection plate under a high temperature. The distorted reflectionplate functions as a concave mirror to concentrate heat or as a convexmirror to disperse the heat. Accordingly, it is considered that theuniform temperature distribution can be obtained when using ceramicswhich are less susceptible to distortion.

The aforementioned results show that even the temperature of the backsurface of the stainless steel plate 4 can be suppressed to about 230°C., which means that the casing 2 can be suppressed to a further lowertemperature. This proves the effectiveness of the stainless steel plate4.

Effects of the first embodiment will be detailed below.

(a) In the first embodiment, the stainless steel plate 4 having thereflection plane S1 is arranged between the casing 2 and the heater 3with a predetermined space distance L1 from the heater 3. Accordingly,the radiant heat from the heater 3 is reflected by the reflection planeS1 of the stainless steel plate 4 and is returned to the heater 3. Thissubstantially reduces the thermal energy loss from the heater 3. Thatis, the thermal energy loss is significantly reduced. Consequently, itis possible to effectively increase the temperature of the heater 3 ascompared to a conventional apparatus having no reflection member.

On the contrary, when the stainless steel plate 4 is used, the radiantheat amount reaching the casing 2 is assured to be reduced, whichenables prevention of an excessive temperature increase of the casing 2.Moreover, since the stainless steel plate 4 is not in direct contactwith the heater 3, it is also possible to prevent temperature increaseof the stainless steel plate 4 caused by the heat conduction.

As has been described above, in the first embodiment, it is possible torealize the hot plate unit 1 having a heater whose temperature can beincreased in a short period of time. This reduces a predetermined timerequired for the entire wafer drying step, thereby improving thesemiconductor productivity. Moreover, it is possible to preventtemperature increase of the casing 2 exceeding the temperatureresistance of the metal material of the casing 2.

(b) In the first embodiment using the stainless teal plate 4, it ispossible to obtain the reflection function. Accordingly, there is noneed of arranging a heat-insulting material between the heater 3 and thecasing 2 for preventing radiant heat. Accordingly, no dusts aregenerated. This enables obtaining of the hot plate unit 1 suitably usedin the semiconductor production field which requires a highly cleanenvironment.

(c) The heater 3 of the first embodiment uses the plate-shaped member 9in the form of a disc made from nitride ceramics having an excellentheat resistance, a smaller thermal expansion coefficient than metals,and a high thermal conductivity. Accordingly, the heater 3 can have asmall thickness and light weight and exhibits an excellent temperaturecontrol characteristic.

It should be noted that the present invention is not limited to theaforementioned embodiment but can be modified to other embodiments asfollows.

As shown in FIG. 6, a hot plate unit 21 according to another embodimentmay have a plurality of (two in FIG. 6) stainless steel plates 4 servingas the plate-shaped reflection member. In this case, it is possible toimprove radiant heat shielding action and to reduce the heatertemperature increase time.

Instead of the stainless steel plate 4 of the first embodiment, it ispossible to use a metal plate made from at least one selected from agroup consisting of a copper plate, a nickel plate, an aluminum plate,and an iron plate; at least one ceramic plate selected from a groupconsisting of oxide ceramics, carbide ceramics, and nitride ceramics;and at least one resin selected from a group consisting of polyimideresin, epoxy resin, and fluorine resin. These can also be used incombination with the others.

As the oxide ceramics, it is possible to use at least one selected froma group consisting of alumina, silica, cordierite, and mullite. As thecarbide ceramics, it is possible to use at least one selected from agroup consisting of silicon carbide, titanium carbide, molybdenumcarbide, tungsten carbide, and the like. As the nitride ceramics, it ispossible to use at least one selected from a group consisting ofaluminum nitride, silicon nitride, and titanium nitride.

Resin and ceramics have thermal conductivity lower than metals and heatabsorption amount smaller than metals. Accordingly, in such a reflectionplate, the temperature of the back surface of the reflection plate isnot easily increased as compared to a reflection plate made from ametal.

Moreover, resin and ceramics are basically thermal insulators.Accordingly, it is possible to fix the lead line connected to a heatgenerating circuit and a line leading from the thermocouple (temperaturemeasuring element) to the reflection plate. In this case, it is possibleto prevent short-circuit due to contact between a lead line with anotherlead line.

In the hot plate unit using the resin plate, if the thermocouple isunintentionally removed and it becomes impossible to control the heatgeneration, the resin plate is burnt down. This cuts off the lead lineconnecting the power source to the heat generation member, therebypreventing further heat generation. That is, this resin plate functionsas a thermostat.

Since the ceramic reflection plate is not distorted by heat, in the hotplate unit using the ceramic reflection plate, it is possible to preventlocal concentration or dispersion of heat caused by distortion of thereflection plate. Accordingly, the hot plate unit having the ceramicreflection plate exhibits an excellent temperature uniformity on theheating surface.

FIG. 7 shows another example of the hot plate unit 31, in which thereflection member is made from a metal material in the form of foil suchas an aluminum foil 32. In this case also, the reflection plane S1should be formed on at least one side of the aluminum foil 32. The metalfoil may be other than aluminum, such as metal foil made from gold,silver, nickel, and the like. It should be noted that it is alsopossible to use the plate-shaped reflection member of the firstembodiment in combination with the foil-shaped reflection member of FIG.7.

FIG. 8 shows still another example of the hot plate unit 41, in whichthe reflection member is made from a layered material such as acopper-plated layer 42. The plated layer 42 may be formed on the innerwall of the casing 2. The plated layer 42 may be formed from a metalother than copper such as gold, platinum, silver, aluminum, chromium,nickel, cobalt, and the like. In this case, the plated layer should havea surface roughness in a range facilitating the reflection plane S1. Asfor the radiant heat shielding action, however, the reflection member ofthe first embodiment and the example of FIG. 7 have bettercharacteristic because the reflection member is arranged apart from thecasing 2.

The present invention is not limited to the heat generating wiring layer10 buried between layers of the heater 3, i.e., a so-calledhigh-temperature heater but can also be applied to a so-calledlow-temperature heater in which the heater 3 is burnt to be attached tothe outer surface of the plate-shaped member 9. In this case, it isdesired that the surface of the heat generating wiring layer 10 iscoated with a metal layer so as to prevent oxidization.

What is claimed is:
 1. A hot plate unit comprising: a) a casing havingan opening portion and a bottom; b) a heater positioned on the openingportion, said heater including a plate-shaped member made from ceramicsand a heat generating element positioned on the plate-shaped member; andc) a plate-shaped reflection assembly interposed between the casing andthe heater and distanced from the bottom of the casing and the heater,whereby the assembly does not contact the heater.
 2. The hot plate unitaccording to claim 1, wherein the plate-shaped reflection assembly is atleast one selected from the group consisting of a metal plate, a ceramicplate and a resin plate.
 3. The hot plate unit according to claim 1,wherein the ceramics are compounds selected from the group consisting ofnitride ceramics and carbide ceramics.
 4. The hot plate unit accordingto claim 1, wherein the plate-shaped reflection assembly is positionedparallel to a back surface of the heater.
 5. The hot plate unitaccording to claim 4, wherein the plate-shaped reflection assemblycomprises a plurality of plate-shaped members.
 6. The hot plate unitaccording to claim 4, wherein the plate-shaped reflection assembly hasat least two surfaces and said surfaces have reflection planespositioned thereon.
 7. The hot plate unit according to claim 1, whereinthe reflection assembly includes a foil reflection component.
 8. The hotplate unit according to claim 1, wherein the plate-shaped memberincludes a plurality of plate-shaped components, and at least one heatgenerating element is interposed between at least one pair ofplate-shaped components.
 9. A hot plate unit comprising: a casing havingan opening portion; a heater positioned on said opening portion andincluding a plate-shaped member made from ceramics and a heat generatingelement positioned on the plate-shaped member; and a plate-shapedreflection assembly interposed between the casing and the heater,wherein the casing and the heater are positioned at a distance from eachother, whereby the assembly does not contact the heater.
 10. The hotplate unit according to claim 9, wherein the plate-shaped reflectionassembly is positioned parallel to a back surface of the heater.
 11. Thehot plate unit according to claim 9, wherein the plate-shaped reflectionassembly is at least one selected from the group consisting of a metalplate, a ceramic plate and a resin plate.
 12. The hot plate unitaccording to claim 9, wherein the ceramics are compounds selected fromthe group consisting of nitride ceramics and carbide ceramics.
 13. Thehot plate unit according to claim 9, wherein the plate-shaped reflectionassembly comprises a plurality of plate-shaped members.
 14. The hotplate unit according to claim 9, wherein the plate-shaped reflectionassembly has at least two surfaces and said surfaces have reflectionplanes positioned thereon.
 15. A hot plate unit comprising: a casinghaving an opening portion; a heater positioned on said opening portionand including a plate-shaped member made from ceramics and a heatgenerating element positioned on the plate-shaped member; and aplate-shaped reflection assembly interposed between the casing and theheater, wherein the heater has a plurality of terminal pins electricallyconnected to the heat generating element, and a plurality of dummy pinswhich are longer than the terminal pins and which are not electricallyconnected to the heat generating element.
 16. The hot plate unitaccording to claim 15, wherein the heater comprises a heat-resisting andinsulating sleeve which encases the terminal pins.
 17. A hot plate unitcomprising: a casing having an opening portion and a bottom; a heaterpositioned on the opening portion, said heater including a firstplate-shaped member made from ceramics and a heat generating elementpositioned on the first plate-shaped member; and a second plate-shapedmember interposed between the casing and the heater and distanced fromthe bottom of the casing and the heater, whereby the second plate-shapedmember does not contact the heater.
 18. The hot plate unit according toclaim 17, wherein the second plate-shaped member is at least oneselected from the group consisting of a metal plate, a ceramic plate,and a resin plate.
 19. The hot plate unit according to claim 17, whereinthe ceramics are compounds with at least one compound selected from thegroup consisting of nitride ceramics and carbide ceramics.